Friday, September 4, 2015

The ancestral squamate was oviviparous ― I think

re-evolved egg-laying from a viviparous ancestor. Right the viviparous Kenyan

sand boa Eryx
colubrinus. Photo
credits: Rick Staub and Arkive, and

Roy Stockwell.

I very much dislike chicken and egg
questions because it immediately sucks you into an argument with a creationist, most of whom simply don't get it. As Wright et al. (2015) point out the answer is clear from an evolutionary standpoint.
The amniote egg, existed by the time the earliest amniotes (mammals and
reptiles) diverged from one another about 325 million years ago, long before
the first chicken (= birds) walked the Earth. Today, the majority of living amniotes
are oviparous, including all birds, crocodylians, tuataras, turtles, and
monotreme mammals. However, squamates are far more diverse in their approach to
giving birth or laying eggs.

Approximately 20% of squamate species
are viviparous, and this complex of traits has been estimated to evolve
independently over 100 times across the squamate phylogeny. Viviparous species,
developing embryos are retained in the mother's uterus for the entire duration
of embryonic development. The traditional view of laying eggs or giving birth in
amniotes is that the most recent common ancestor of squamates, which lived ~200
mya, was oviparous, as it inherited the same ancestral parity mode that characterizes
all other reptiles.

The transition from oviparity to
viviparity requires extensive modification of uterine physiology and morphology.
For example, uterine shell glands in oviparous species secrete calcium during
the discrete period of eggshell construction. In viviparous species, shell gland
function has been modified to provide calcium to the embryo throughout
gestation. True “ovoviviparity” does not exist in squamates, as all examined
viviparous squamates have some form of placenta composed of both maternal and
embryonic tissue.

The uterine structure of oviparous
species is, therefore, modified into the maternal half of the placenta in
viviparous species. The embryonic portion of the placenta is composed from the
same extra-embryonic membranes that are present in all amniote eggs. Most
squamate placentae are relatively simple structures used primarily for gas
exchange and water transport, but a more elaborate placenta that facilitates significant
nutrient exchange has evolved at least six times in squamates. Underlying the
evolutionary transition to viviparity and a placenta are significant changes in
gene expression of hundreds of genes.

In a new paper Wright et al. (2015) re-evaluate support for the
provocative idea that the first squamates were viviparous. They test the
sensitivity of the analysis to model assumptions and estimates of squamate
phylogeny. They found that the models and methods used for parity mode
reconstruction are highly sensitive to the specific estimate of phylogeny used,
and that the point estimate of phylogeny used to suggest that viviparity is the
root state of the squamate tree is far from an optimal phylogenetic solution.

The ancestral state reconstructions
are also highly sensitive to model choice and specific values of model
parameters. A method that is designed to account for biases in taxon sampling
actually accentuates, rather than lessens, those biases with respect to
ancestral state reconstructions. In contrast to recent conclusions from the
same data set, Wright et al. (2015) found that ancestral state reconstruction
analyses provide highly equivocal support for the number and direction of
transitions between oviparity and viviparity in squamates. Moreover, the
reconstructions of the ancestral parity state are highly dependent on the
assumptions of each model. The authors conclude that the common ancestor of squamates
was oviparous, and subsequent evolutionary transitions to viviparity were
common, but reversals to oviparity were rare. The three putative reversals to
oviparity with the strongest phylogenetic support occurred in the snakes Eryx jayakari and Lachesis, and the lizard, Liolaemus
calchaqui. The authors emphasize that because the conclusions of ancestral
state reconstruction studies are often highly sensitive to the methods and
assumptions of analysis, researchers should carefully consider this sensitivity
when evaluating alternative hypotheses of character-state evolution.

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